This application proposes 1) to identify the enzymatic activities required for the somatic rearrangement of immunoglobulin genes, 2) to analyze the organization and rearrangement of diversity (D) gene segments of immunoglobulin heavy chairn genes and 3) to characterize the in vitro system of heavy chain class switch, mediated by mitogens or by cell free factors. Identification of the recombinase and establishment of the invitro recombination system will enable us to study the molecular basis of V-J joining and of switch recombination. Characterization of both germline and rearranged D segments is particularly important to investigate the diversity generation of immunoglobulin heavy chain genes. Analysis of immunoglobulin heavy chain genes in the in vitro switch system will provide new insights into the simultaneous expression of multiple heavy chain isotypes and heavy chain switching. These studies on murine immunoglobulin genes will shed light on the basic mechanisms of human immune diseases including heavy chain abnormalities and defects in lymphocyte differentiation.